However, this fusion penalty \(\Omega _{\lambda _2}(\psi )\)  still complicates optimization, so next we use Nesterov smoothing to substitute it with a smoothed, differentiable surrogate [1]}. Defining \(D_{\lambda _2}\)  as a contrast matrix such that \(\Omega _{\lambda _2}(\psi ) = \Vert D_{\lambda _2}\psi \Vert _1\) , the surrogate is
\({\widetilde{\Omega }}_{\lambda _2,\mu }(\psi ) = \max _{\Vert \mathbf { z}\Vert _{\infty }\le {} 1}\left( \mathbf { z}^{{\mathsf {\scriptscriptstyle T}}}D_{\lambda _2}\psi - \mu \frac{1}{2}\Vert \mathbf { z}\Vert ^2_2 \right) = (\mathbf { z}^*)^{{\mathsf {\scriptscriptstyle T}}}D_{\lambda _2}\psi - \mu \frac{1}{2}\Vert \mathbf { z}^*\Vert ^2_2,\) 
